Windows having low emissivity (low e) find particular use in cold climates for preserving the heat in homes, offices, automobiles and other heated environments and for mitigating escape of warm room air to the cold exterior via the window glass. Low e windows are also useful in hot climates for rejecting thermal energy radiation from the exterior and for minimizing thermal energy transfer via the windows between the interior and the exterior, thereby to maintain a cool interior.
Window glass itself may be treated and/or coated to provide low emissivity characteristics. However, treated glass does not provide adequate protection against ultraviolet (UV) radiation or adequate mechanical protection for the metal or other material coated on glass. Also, if the treated glass becomes corroded or is broken or otherwise damaged, it is very difficult to match the appearance and/or color of a new or replacement glass with the appearance and color of the original glass.
A more practical approach is provided by flexible polymeric films that can be adhered to window glass. Such films are in widespread use and provide a variety of solar control functions. The films are easy to apply, can conveniently be removed and replaced, and can readily be made to duplicate the color and appearance of the film and/or film covered glass that is being replaced. Also, flexible films facilitate retrofitting of existing clear glass window panes to impart solar control functions to the same.
The majority of solar control films are made by metalizing a polymeric substrate film, usually poly(ethylene terephthalate) (PET), and then laminating a second film of PET onto the metalized surface of the substrate film. This results in a product having high emissivity, e.g., an emissivity in the order of about 0.7.
At the time of this writing there is, in the commercial market, one polymeric "winter" film product having low emissivity, specifically an emissivity of about 0.35. However, in order to attain such low e, the visible light transmission (VLT) of the film must be reduced to about 20%. Also, the film has little resistance to scratching and mechanical damage and must be handled, installed and cleaned with extreme care.
As shown in the accompanying FIG. 1, the low emissivity commercial product is comprised of a transparent flexible polymeric substrate bearing a transparent but relatively thick and dense layer of aluminum and a sheet of oriented polypropylene (OPP) that is laminated to the metalized surface of the substrate. The OPP has relatively low absorption in the thermal radiation range and the aluminum is sufficiently reflective to long wavelength infrared radiation (IR) that the film has low e. However, the OPP sheet is not free of absorption, and the laminating adhesive used to laminate the OPP sheet to the metalized substrate is also absorptive to thermal radiation. Consequently, the aluminum layer must be made dense and thick in order to hold the emissivity of the composite film to a value as low as 0.35, at which value VLT is no greater than about 20%. Also, the OPP has little if any resistance to scratching or mechanical damage. Any attempt to increase either scratch resistance or VLT results in a sharp increase in the product's emissivity, and a diminution in its ability to serve its intended function.
There is, therefore, a demand in the industry for window films having lower emissivity, greater visible light transmittance, and greater resistance to scratching and mechanical damage. Such films are especially desired for retrofitting of clear window panes.